style: make rustfmt use the default configuration (starship#1661)

by adding an empty `rustfmt.toml` configuration file, thus allowing
users who have their own configuration to contribute without friction.

Author: tapeinosyne

.rustfmt.toml

@@ -0,0 +1,5 @@
+# This file intentionally left almost blank
+#
+# The empty `rustfmt.toml` makes rustfmt use the default configuration,
+# overriding any which may be found in the contributor's home or parent
+# folders.

CONTRIBUTING.md

@@ -95,7 +95,7 @@ cargo clippy
 
 ## Formatting
 
-Starship source files are formatted with [rustfmt](https://crates.io/crates/rustfmt-nightly). Rustfmt will be ran as part of CI. Unformatted code will fail a build, so it is suggested that you run rustfmt locally:
+Starship source files are formatted with [rustfmt](https://crates.io/crates/rustfmt-nightly), using the default configuration. Rustfmt will be ran as part of CI. Unformatted code will fail a build, so it is suggested that you run rustfmt locally:
 
 ```sh
 rustup component add rustfmt